In the construction of roads, grades and dams, the filling masses or base courses are packed to a suitable density and carrying capacity. If the compacted surface is to be asphalted, the laid asphalt must also be compacted. In this type of compacting work it is appropriate to use rollers which are equipped with one or more vibrating drums. The compacting work supplied during one pass with a roller of a certain weight class and vibrating mass depends largely on the amplitude with which the drum is vibrated and the frequency at which the vibrations occur. In compacting work using such vibrating rollers, it has been shown to be advantageous to control the amount of compacting work supplied by regulating the vibration amplitude of the drum at a fixed frequency. During the first few passes, applying the maximum vibration amplitude is recommended, and during the final passes, when the subgrade begins to become finish-compacted, a lower amplitude is applied. If the hard subgrade that is almost finish-compacted is vibrated at too high an amplitude, the roller tends to “bounce” which adversely affects its mechanics and may also give rise to undesirable loosening of the surface layer. If the almost finished packed subgrade consists of asphalt, there is a risk that the constituents of the asphalt will be crushed, thereby reducing the quality of the asphalt covering.
These are some of the reasons why roller manufacturers want to equip their rollers with drums in which the vibration amplitude can be varied by influencing the eccentric shafts of the drums. The most common method is to fit the rollers with eccentric shafts the eccentric moment of which can be varied. The eccentric moment refers to the product of the unbalanced mass of the eccentric shaft and distance of the center of gravity thereof from the center of rotation of the shaft. The variable eccentric shafts are often based on two tubes arranged coaxially and fitted with eccentric weights which can be turned relative to each other by means of the turning devices on the eccentric shaft. When the weights balance each other out, this enables a minimum eccentric moment to be obtained and when the weights interact, a maximum eccentric moment can be obtained. The turning device is actuated by axial regulating forces which are transformed by the turning device into turning movements.
In order to generate axial regulating forces, an apparatus is required comprising an adjusting device and a force transmission mechanism. The adjusting device, in which the axial regulating forces are generated, is located outside one of the drum heads. The function of the force transmission mechanism is to conduct the regulating forces to the turning device located inside the drum. This invention relates to such an apparatus.
The adjustable eccentric shafts described in Austrian patent publication 375,845 and Swedish patent publication 514,877 disclose the adjustable eccentric shaft described above. In Austrian patent publication 375,845, an eccentric shaft with turnable eccentric weights, actuated by a turning device, is described. The turning device and the adjusting device are arranged at a certain distance from each other and are connected by a rod. The rod may be said to constitute the aforementioned force transmission mechanism, except that one of its ends constitutes the piston in a single-acting hydraulic adjusting device with which the axial regulating force is generated. The restoring force is generated by a helical spring.
In practical tests, the applicant has noticed that minor, commonly occurring variations in hydraulic pressure give rise to considerable variations in regulating force on this type of hydraulic adjusting device. The result is unacceptable variations in the vibration amplitude. Moreover, providing space for a sufficiently strong helical spring is too complicated within the available area. There is also a problem in finding a reliable method of reading the instantaneous position of the hydraulic adjusting device in the regulating range. The force transmission mechanism (rod) of the eccentric shaft of the prior art runs coaxially through the drive shaft center of the eccentric shaft. As a result of this, the hydraulic pressure must be supplied in a complicated manner to the actuating device via the drive unit of the eccentric shaft.
The adjustable eccentric shaft disclosed in Swedish patent publication 514,877 shows, in several embodiments, how the hydraulic pressure can be supplied by simpler means. In this eccentric shaft, the drive shaft is arranged in the center of a hydraulic adjusting device and the force transmission mechanism comprises two or more actuating rods which are located parallel and symmetrically around the center of the eccentric shaft. One of the embodiments shows how the necessity for the helical spring can be eliminated by making the hydraulic adjusting device double-acting.
However, the aforementioned problems relating to variations in hydraulic pressure and position determination are not solved in these embodiments. One of the embodiments in Swedish patent publication 514,877 shows how the problem of variable hydraulic pressure can be solved by changing to a mechanical adjusting device. The mechanical adjusting device is based on a worm gear and actuates a force transmission mechanism comprising only one actuating rod. The drive shaft of the eccentric shaft runs through the center of the adjusting device and has been provided with axial slots for operating the adjusting device. The applicant has gained practical experience of the drive shafts of eccentric shafts being subjected to extremely high fatigue stresses. Stress concentrations, which may very well lead to fatigue failure, occur around an axial slot of this type.
According to the applicant, a freestanding drive shaft without grooves for the operation of the adjusting device is preferable for this reason.
Another problem that requires unconventional mechanical solutions is the perpendicular orientation and position of the worm gear relative to the eccentric shaft. The space for the adjusting motor of the adjusting device in this direction is very limited in a roller drum, while, at the same time, the required connecting flange of the adjusting motor tends to collide with the connecting flange for the eccentric shaft drive motor. It is more advantageous to arrange the adjusting motor in parallel with the drive motor.